Using dynamic paint images for displacement
This is an "answer" only because i wanted to attach an image.  To ask a basic question, have you tried manually editing your depth image so the contrast is more pronounced?  A quick and dirty version of that is below.  I am guessing you are not going to see noticeable results from many of the methods you describe if the color differences are too subtle. 



If that fixes the problem, then I guess there are several ways you might increase the contrast in your images, some within blender and some external.  It sort of depends on how you're using it, for example if you're using a bump map you might try a brightness/contrast node.